The tkinter.tix module looks for a Tix installation in the directory specified by the TIX_LIBRARY environment variable, but blindly trusts that it is a path in the filesystem. This enables a shell injection :

TIX_LIBRARY='/dev/null}; exec gsimplecal;' python2 -c "from Tix import Tk; Tk()"

or 

TIX_LIBRARY='/dev/null}; exec gsimplecal;' python3 -c "from tkinter.tix import Tk; Tk()"

Python execs gsimplecal, waits on its completion and then raises a tkinter.TclError.

The offending code is here : https://github.com/python/cpython/blob/master/Lib/tkinter/tix.py#L204-L208